There will be four types of laboratory reports in the course. Two formal or "long reports" will be required during the semester. Most of the reports will be in what we call the "short report" format. One of your reports will be an oral presentation, usually as part of a small group effort; one report will include a poster presentation. In addition, there will be several other assignments which have their own peculiar format.
The reports should be text edited and printed, although equations, sample calculations, and special symbols may be written by hand. You are expected to pay careful attention to spelling and we strongly recommend the use of spell-checking software before you print the final copy. You are also expected to write clearly and to use proper grammar and vocabulary.
The "short reports" are to be written with the assumption that the instructor or another person in the class is the intended reader. You can assume that we have seen the experiment and know the laboratory procedures. You do not need to explain theory or describe the experimental procedures. In most instances the report will have the following sections.
The data will often appear in the form of tables, with suitable identifying headings and units. You will generally find that spreadsheets are a convenient method to prepare this section. The data section should also include a brief list of experimental conditions (solution concentrations, temperature, instrument settings, column used, etc.) This is true even if you are following the laboratory notes precisely. You must include the basic (raw) data in sufficient detail that your results can be independently checked. If you use a spreadsheet include sufficient annotation (comments, labels) that the contents will be clear to the intended reader.
This may be part of the tables generated within a spreadsheet; if so, clearly explain what is being done. Include basic formulae and parameters you use in the calculations. Where appropriate, include a sample calculation. In many cases the data is used to produce graphs and you use curve fitting techniques (such as regression) to extract information like slopes and intercepts.
Some portions of a report are fairly easy to organize. Later in the year you will measure the IR spectrum of HCl gas to determine the H-Cl bond length. A copy of the spectrum, neatly labeled is a pretty obvious component of the report. A table of the wavelengths (or more likely the wavenumbers) of the peaks is needed, since this is the raw data from which all calculations are made. The computational scheme involves creating two graphs from this table; the bond length is computed from the slope of these two graphs. A few lines should suffice to go from the slope to the numerical value for bond length (the result.)
In most cases you will use a computer spread sheet to handle the calculations. The spreadsheet can draw the graph and it can perform a regression analysis (find the best straight line and evaluate the slope.) It is often easy to edit a copy of the spreadsheet and incorporate it directly into your report. You will almost certainly need to provide some additional annotation and rearrangement to turn the spreadsheet output into a coherent part of a report, but there's seldom a need to retype data tables.
This will involve some professional judgment. If you set out to measure the lead content of a water sample, the result might be as simple as "the sample we were given contained 1.72 +0.07 ppm of lead." It's possible there are no other conclusions to be drawn. If the sample were presented as drinking water, you should probably look up acceptable lead levels and comment on the significance. In some instances, like the bond length of HCl, there will be accepted literature values for comparison.
Some "experiments" are clearly demonstrations of instrumentation and techniques. It may be appropriate to make comparisons with other methods. If you encountered difficulties, there may be some appropriate remarks here. (However, don't overdo it with lines like "due to our lack of familiarity with the equipment"...)
A numerical result without some sense of reliability is basically a worthless value. We spend a lot of money for well designed instruments and we often take elaborate precautions to build calibration into our work so that we have a realistic sense of how much we can trust the numbers. We usually repeat our determinations to gain some statistical sense of the reliability of our results. You must, at each stage of a report, convey this kind of information. In some instances, this is done by the number of significant figures for the data and by statements about the accuracy of the materials and the measurements.
All reports (long and short) should contain an abstract. This is a concise statement of what you studied, how you made your measurements, and what you found. This should be done in 3-5 sentences. You should assume that the reader has a BS in Chemistry, and you should assume that standard abbreviations (NMR) and methods (spectrophotometry) can be cited without explanation. You should not assume that the reader is familiar with the course content or the furnishings of the laboratory.
Think of this abstract as something which would be made available to someone who is hunting for information on one of your topics. That person should be able to read the abstract and decide if your report contains details which are likely to be of value. Remember that the abstract must convey information in the absence of the rest of the report.
This can be the most difficult part of the report since you must decide what level of detail is required and how far back you wish to go in developing the theory. You also need to set the work into a context. It is important that this material not just be a simple rewriting of what is in the laboratory notes (handouts.) It is presumed that you will look in several references to help you understand and write this section.
This is also a section which calls for professional judgments. While you certainly need to list any chemicals species and solution s which are used, you don't need to describe how simple solutions were prepared.
One convenient piece of shorthand is the phrase "using standard volumetric glassware." This would mean that solutions are prepared using class A volumetric flasks and any dilutions are prepared using class A transfer pipettes. Some other useful phrases are
You should feel free to ask for help, especially in establishing the proper level of detail. You may bring in a draft of the report for review without grading. You may work with other students, especially your lab partner, as you prepare your reports and the data sections will often be identical spreadsheet printouts. The text should be your own work, developed independently.
Under some circumstances your report may include data from other student groups. This may be a supplement to the data you collected or an admission that your data has serious problems. Such data must be clearly cited.
The dates when reports are due will be announced and posted in the lab. Late reports are subject to penalty unless I have agreed to extensions.
EXPERIMENTAL
Reagents: The Isopropanol and Acetone were obtained from the laboratory stock bottles and were used without future treatment. These samples were identified as repackaged reagent grade material.
Apparatus: A Bausch and Lomb Model 101 refractometer was used; it was connected to a circulating constant temperature bath at 25 C (+/- 0.5 degrees.) This particular instrument has a damaged prism, making it difficult to read the refractive index to better than .001 even though the manufacturer suggests it can be read to 0.0002. A calibration curve was used to interpret the readings; no attempt was made to determine absolute calibration of the instrument or to correct readings to the standard 20 C.
The samples were refluxed in a custom built glassware. The apparatus consisted of a 100 ml round bottom flask with a sidearm which held a vertically mounted condenser. Condensed vapor was returned to the flask though this sidearm, although a small sample of liquid (1-2 ml) collected at the bottom of the sidearm. Samples of this condensed liquid (approx. 1 ml) were withdrawn and stored in capped 1 ml bottles for analysis. A mercury thermometer in the neck of the flask was used to record the boiling point.
CALIBRATION: Ten samples of varying isopropanol/acetone were prepared and used to produce a refractive index calibration curve. Samples (1-10 ml of each liquid) were weighed to the nearest 0.002 grams. A second order polynomial was determined by polynomial regression; for convenience the fit was mole fraction of isopropanol vs refractive index and the expansion was performed around the midpoint of the refractive index range for the samples. (A expansion about zero is not convergent.)
REFLUXING PROCEDURE: One of the flasks was filled with 25 ml of isopropanol. The liquid was brought to a boil and a steady temperature was observed; a sample of the condensed liquid was collected from the reflux trap and was stored in a 25 ml capped bottle. Then 2 ml of acetone was added using a volumetric pipette and the process was repeated. Each sample was refluxed for five minutes after a steady temperature reading was obtained. This procedure continued until 20 ml of acetone had been added. A second flask was used with 25 ml of acetone and similar additions of isopropanol. All samples were analyzed on the same day they were collected.
ERROR ANALYSIS
1. The refractive index calibration showed a mean standard error of 0.023 in the mole fraction. The experimental uncertainty in refractive index (0.001) corresponds to approximately 0.03 uncertainty in mole fraction.
2. The temperature could be estimated to 0.2 C; the thermometer was marked at 1 degree intervals. The thermometer calibrations were not checked. Catalog descriptions of similar thermometers (new) indicate a tolerance of about 0.2o C. This means that the two thermometers used might have 0.3-0.4 degree differences. A potentially more serious source of error might be the lack of insulation about the neck of the flask, leading to a lower temperature at the location of the thermometer. No estimate of this error was made. The thermometer is calibrated with a 75 mm immersion depth; in the apparatus the heated vapor covers approximately 50-60 mm of the stem. No correction for immersion depth was made but catalog entries suggest the error is less than 0.02 degrees (ref 1). The boiling points of the pure liquids differ by 0.2 and 0.7 degrees from the accepted values; see also items 3 and 4 below for alternative interpretations of these values.
3. Both isopropanol and acetone have a tendency to absorb moisture from the air and supplies are often contaminated with water vapor. The presence of water would result in a three component mixture and a corresponding error in the calibration curves. No attempt was made to test for this possibility or to estimate its effect on the results.
4. Another error is that the barometric pressure was not recorded during the experimental work and a pressure of 760 torr was used, by default, in the calculations.
5. The most serious source of error identified in this work is the assumption that the composition of the liquid phase can be determined from the volume of the two liquids which had been added to the flask. This calculation ignores completely the volume of liquid withdrawn as samples (up to 10-15 ml in the later stages of the procedure) and it ignores the changes in liquid composition due to the sample which is in the condenser and trap. An estimate of this error is shown in table II.
DISCUSSION: The experimental phase diagram clearly shows the presence of a low boiling azeotrope with a temperature of 68-69 C. The diagram shows a very rapid drop in temperature as the second component is added. The phase diagram shows a very flat region in the x(isopropanol)= 0.3-0.7 region so it is difficult to determine the composition of the azeotrope from this curve. The data table permits a less ambiguous value. Liquid samples with X(i)< X(az) will show a vapor which is richer in isopropanol; samples with X(i)>X(az) will show a vapor weaker in the azeotrope. Entries 8,9, and 14 in the table indicate that this relationship between liquid and vapor changes between x(i)=0.48 and x(i)=0.53. This unfortunately makes the result depend critically on the composition of those three samples, so one is advised to take the assignment of composition (Xi=0.48-0.53 to be tentative.
The activity coefficients of the two components was also evaluated as a function of composition. Dalton's law of partial pressure was assumed to be valid for the vapor, so the pressure of each component was taken to be the mole fraction in the condensed vapor times atmospheric pressure (760 torr.) The vapor pressure of the pure liquid was evaluated using the Clausius Claperyon equation. Data for the boiling point of the pure liquid at 760 and at 400 torr (ref 2) was used to determine the heat of vaporization. The activity coefficient was determined as the ratio of the observed vapor pressure (based on the assay of condensed vapor) to that predicted for an ideal solution (vapor pressure of pure liquid times mole fraction in the liquid.) As expected in a low boiling azeotrope the activity coefficients were significantly greater than unity, quickly dropping to unity near mole fractions of one for that component.
This mixture has been reported in the literature (ref. 3) to have an azeotrope with a composition of x(iso)=0.xx and a boiling point of 68.2 C. The phase diagram and activity coefficient graphs are qualitatively similar to those obtained in the present study.
09/13/93